1. Field of the Invention
The present invention relates to a high chromium ferritic heat-resistant steel. The heat resistant steel of the present invention has excellent long-term creep strength at elevated temperature, high resistance to steam oxidation, and prominent toughness at room temperature. Moreover, the welded joints including heat affected zone (HAZ) have also excellent long-term creep strength and prominent toughness. Therefore, the heat resistant steel of the present invention is suitable for use as a material of equipments operated under conditions of high temperature and high pressure, such as boilers, nuclear power plants, and chemical engineering facilities. More particularly, the heat resistant steel of the invention is advantageously used for making steel tubes for heat exchangers, steel plates for pressure vessels, and turbine parts.
2. Description of the Related Art
Equipments such as boilers, nuclear power plants, and chemical engineering facilities are used under conditions of high temperature and high pressure for long periods. Therefore, heat-resistant steels used for these equipments must have sufficient strength, anti-corrosion properties, and resistance to oxidation at elevated temperature, as well as toughness at room temperature. Moreover, in view of the fact that these steels are also used as structural steels fabricated by welding, the welded joints of the steels must have sufficient long-term creep strength at elevated temperatures and toughness.
For these uses, austenitic stainless steels (such as JIS-SUS321H and JIS-SUS347H steels), low-alloy steels (such as JIS-STBA24 (2 1/4 Cr-1Mo)), and high chromium ferritic steels of a 9-12 Cr series (such as JIS-STBA26 (9Cr-1 Mo)) have heretofore been used. Among them, high chromium ferritic steels are superior to low-alloy steels in terms of strength and anti-corrosion properties in a temperature range from 500.degree. to 650.degree. C. Moreover, high chromium ferritic steels have advantages that they are less expensive than austenitic stainless steels, that they have excellent resistance to thermal fatigue and scale exfoliation due to their high heat conductivities and small heat expansion coefficients. Another notable advantage is that they do not cause stress corrosion cracks. Due to the above-mentioned excellent properties, high chromium ferritic heat-resistant steels have wide applications.
In recent thermal power plants, steam conditions in boilers are becoming higher temperatures and higher pressures in order to achieve high thermal efficiency. In fact, there is a plan to operate thermal power plants under ultra-super critical conditions of 650.degree. C. and 350 atm in the future rather than current super critical conditions of 538.degree. C. and 246 atm. As steam conditions become severe, greater performance is required for steel tubes for boilers. Thus, there is a situation where previous high chromium ferritic steels can no longer satisfactorily meet the above-described requirements due to the shortage of long-term creep strength at elevated temperatures and resistance to oxidation, and particularly resistance to steam oxidation. In this connection, steam oxidation is a phenomenon which occurs on the inside surface of steel tubes of boilers that are exposed to steam of high temperature and high pressure. When scales are generated as a result of oxidation, they tend to come off as the boiler temperature changes. Since exfoliated scales cause severe problems such as plugging of steel tubes, prevention of steam oxidation is an important technical subject.
Austenitic stainless steels have properties that meet the above-described severe requirements. However, their high price limits their use in commercial facilities. Thus, efforts are directed to improve properties of high chromium ferritic steels that are less expensive than austenitic stainless steels and to expand the range of their applications.
As an improvement of the properties of high chromium ferritic steels, a heat-resistant steel containing tungsten into conventional high chromium ferritic steels was developed. For example. Japanese Patent Application Laid-open (kokai) No. 3-97832 discloses a high chromium ferritic steel which has an increased W content compared with conventional ones, and also contains Cu so as to improve resistance to oxidation at high temperatures. Japanese Patent Application Laid-open (kokai) Nos. 4-371551 and 4-371552 disclose high chromium ferritic steels whose strength at elevated temperatures and toughness are enhanced by containing W and Mo with a properly selected ratio between them, and by also containing Co and B.
These high chromium ferritic steels contain a great amount of W, and therefore, they have excellent long term creep strength at elevated temperatures. W, however, is also a ferrite-forming element as are Mo and Cr, a large content thereof inevitably produces .delta.-ferrite in steel. Consequently, a drawback is caused in which the toughness of the high chromium ferritic steel decreases.
In order to effectively prevent a decrease in toughness, high chromium ferritic steels are processed to have a single phase of martensite. Japanese Patent Application Laid-open (kokai) No. 5-263196, following this approach, discloses a heat-resistant steel having a single phase of martensite obtained by reducing the Cr content. Moreover, high chromium ferritic steels with enhanced toughness have been obtained by an addition of austenite-forming elements, e.g., Ni, Cu, Co, etc., into high chromium ferritic steels (Japanese Patent Application Laid-open (kokai) Nos. 5-311342, 5-311343, 5-311344, 5-311345, and 5-311346).
The high chromium ferritic steels disclosed in Japanese Patent Application Laid-open (kokai) No. 5-263196 have the drawback that they have poor resistance to steam oxidation since Mo, Ni, etc. distract a dense and stable scale layer of the corundum type Cr.sub.2 O.sub.3 formed on steel surface. Moreover, the high chromium ferritic steels disclosed, for example, in Japanese Patent Application Laid-open (kokai) No. 5-311342 contain Ni, Cu, etc. abundantly, and therefore, they have low A.sub.c1 and A.sub.c3 transformation points. This means a small resistance to temper softening, which reduces long term creep strength. In addition, when these elements are present, the structure of oxides primarily composed of Cr.sub.2 O.sub.3 is changed. This also brings about another disadvantage that resistance to steam oxidation decreases. In addition, the high chromium ferritic steels disclosed in the latter publication are poor in strength at welded joints, particularly long term creep strength because of softening at heat affected zones (HAZ).
Japanese Patent Application Laid-open (kokai) No. 2-294452 discloses high chromium ferritic steel in which the amounts of Mn, Ni, Cu, etc. are limited at a low level to prevent .delta.-ferrite formed in heat affected zones in an attempt to enhance toughness in welded joints. Japanese Patent Application Laid-open (kokai) No. 6-65689 discloses heat-resistant steel in which softening of heat affected zones is prevented by dispersion hardening of an oxide such as Ta205. However, the steels disclosed in these publications do not have sufficient long term creep strength at elevated temperatures and toughness that would satisfactorily meet the aforementioned severe requirements.
Thus, development is still desired of high chromium ferritic heat-resistant steels which satisfy the requirements of long term creep strength, toughness, and resistance to steam oxidation under ultra-super critical conditions of high temperature and high pressure, and which, when welded, also satisfy severe requirements called for welded joints including long term creep strength at elevated temperatures and toughness.
The present invention was made in view of the foregoing circumstances, and an object of the invention is to provide a high chromium ferritic heat-resistant steel which has excellent long-term creep strength and high resistance to steam oxidation at high temperatures over 600.degree. C. and prominent toughness at room temperatures. Moreover, the present invention provides a high chromium ferritic heat-resistant steel which, when welded, exhibits excellent long term creep strength at elevated temperatures and toughness at room temperatures in welded joints.